1. Field of the Invention
The present invention relates to a channel equalizer used in a frequency domain and a digital television receiver using the same.
2. Discussion of the Related Art
In a general digital communication system, a transmitter maps digital information such as voice, data and image onto symbols, converts each symbol into an analog signal proportional to an amplitude or a phase corresponding to the symbol, and transmits the analog signal to a receiver through a transmission channel. The signal transmitted to the receiver is interfered with adjacent signal during its passing through the transmission channel of multiple paths, so that the signal is distorted very seriously. In order to restore the original signal from a distorted signal, an equalizer is essentially employed to compensate a channel. In general, the most widely used channel equalizer is a decision feedback equalizer (DFE) that uses LMS algorithm. When signals are received through a multiple path channel, the DFE regards the path through which the signal having the largest energy is received as a main path, and regards the remaining paths as reflection paths through which an inter-symbol interference (ISI) or ghost signals are received. Then, the DFE corrects and extracts the phase and the amplitude of only the signal received through the main path and eliminates the signals received through the remaining paths.
FIG. 1 illustrates a configuration of a general decision feedback equalizer operating in a time domain, that is, a time domain equalizer. Referring to FIG. 1, a feed forward filter 101 removes the affection of the signals (pre-ghost signals) of paths, which are received before the signal of the main path, and a feedback filter 102 removes the affection of the signals (pre-ghost signals) of paths, which are received after the signal of the main path. An adder 105 adds the output of the feed forward filter 101 and the output of the feedback filter 102, and outputs the sum of the outputs to a decision unit 103. The decision unit 103 compares the output signal of the adder 105 with a predetermined reference value to determine the output signal of the adder 105 to be at the nearest signal level. Here, the output of the decision unit 103 is fed back to the feedback filter 102 and the control unit 104.
Accordingly, when the decision unit 103 made an exact decision, noises are eliminated from the output of the decision unit 103 and the output of the decision unit 103 is inputted to the feedback filter 102 again. So, the noise is not amplified and the time domain equalizer shown in FIG. 1 is usually better than the linear equalizers in performance.
Also, if a decision error is negligible, the time domain equalizer can have similar performance to that of the maximum likelihood sequence estimator (MLSE).
However, if channel distortion is too serious, the decision error occurs frequently on the decision value inputted to the feedback filter 102 and the wrong decision value is infinitively looped in the feedback filter 102. So, the time domain equalizer can deteriorate in its performance. This situation is called error propagation situation. If the main path is cut and only the signals passing through the reflection paths exist, or if the same signal is transmitted though different paths from multiple antenna (this network is, so called, a single frequency network (SFN), the energies of the signals received through the paths are similar to one another so that it is unclear which signal of them is the main signal. In other words, in case the locations of a main path and a reflection path in the time domain equalizer are occasionally changed, the time domain equalizer deteriorates in performance and frame synchronization changes frequently so that channel decoding performed in a rear stage of the equalizer is impossible.
In this situation, it is meaningless to distinguish a main signal from reflection signals and the DFE cannot equalize the signals correctly so that the DFE is not proper to multiple path and SFN channel compensation.